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October 23, 2006

A Look at Radiation Therapy for Pets
By Meghan A.T.B. Reese
Radiology Today
Vol. 7 No. 21 P. 30

Like many treatments designed for people, radiation therapy is also finding a role in animal medicine.

Like many before me and many still, I used to ask for a puppy, a pony, or a kitten every Christmas. My sister picked up the torch a few years later. But no fuzzy furball wrapped in a big red bow ever appeared under the tree. When we moved, the pony idea became a farther reach and as my allergies grew worse and worse (probably because I was never exposed to any cuddly kittens!), the mere idea of a cat made my eyes itch.

So this Christmas, I am buying my family a puppy. The puppy is no longer just a big, live toy, “a great opportunity for responsibility,” or any other pitch I would spin. This puppy is to be a new Reese, a reason to get my father outside and exercising for his new knee and a companion to my mother who is suffering empty nest syndrome a little harder than the rest. This new addition is going to be just that—a new addition to our family, and like every other family member, he or she will be expected to uphold a certain demeanor, attend all family functions, love everyone equally, and will be rewarded with a warm bed, daily meals, family vacations, birthday and Christmas gifts, and most importantly, lots of love and care, including healthcare, no matter what.

Pets have long played important roles culturally, socially, and maybe even politically. There is a growing acknowledgement that a human-animal bond can potentially be a strong, lifelong partnership. Why else would there be designer pet gear and pet salons complete with pedicures and massages? Many view their pets as family members. And just like human family members, pets get sick.

Cancer is the No. 1 natural cause of death in geriatric cats and dogs, and it accounts for nearly 50% of pet deaths each year. Although cancer is the leading cause of death in geriatric animals, it’s also the most treatable disease when compared with life-limiting diseases such as congestive heart failure, renal failure, and diabetes.

Despite the need for veterinary radiation and cancer specialists, their numbers are meager.

“We’re small. We’ve just started. There is little over 50 of us that have passed the certification in radiation oncology,” says Lisa Forrest, VMD, DACVR, radiation oncologist at the University of Wisconsin (UW)-Madison School of Veterinary Medicine. “We are following in the footsteps of human radiation oncology. We’re a little behind, but we’re trying to keep up.” Even though they may be a little harder to find, Forrest says, “it’s important for people who are considering radiation therapy for their pets to go to a certified radiation oncologist.”

Similar but Different
Man and man’s best friend may be species apart, but their cancers are not. “There are a lot of tumors that are very similar. Especially in the dog, there are a lot of similarities with human cancer. As in the case of nasal tumors in a dog,” Forrest says, “they are tumors that tend to be locally invasive, when they spread to other parts of the body, it is usually to the local lymph nodes, and generally the best therapy is radiation therapy plus or minus surgery.”

Because some cancers are so similar, your pet could save your life in ways other than sounding the alarm of a fire or saving you from drowning. “I think they provide a good model for new treatment therapies or evaluating things that would then go into clinical trials in humans,” Forrest says. “The life span of a dog is much shorter than a human, so you’re going to get information back at a faster rate. It’s definitely a win-win situation in that we are able to treat our pet dog population and get some information that will help human cancer patients down the line.” UW recently participated in a funded clinical trial for dogs with nasal or paranasal sinus tumors that could one day aid human cancer patients.

Another common trait is risk factors. “There are not quite as many risk factors, but there are some,” Forrest says. “With bladder cancer in pigs, one of the risk factors is obesity. There are also risk factors in dogs with nasal tumors—it has been found to be more prevalent in dogs with longer noses. Cats get cancer of their nose and face that’s secondary to sun exposure similar to humans. It could be genetic in origin or it could just be as cells age they undergo defects in their DNA that make them more susceptible to developing cancer.”

However, a stark difference is preventability. Many risk factors leading to human cancers stem from personal choice. Stay out of the sun and use sunscreen to reduce your chances of developing skin cancer. Lay off the liquor to cut liver cancer risk. Stop or never start smoking and decrease your risk of developing lung cancer. Exercise and pay attention to personal nutrition—obesity is a reported factor in the development and prognosis of breast, uterine, prostate, and colorectal cancers.

But in many pet cancers, the risk factors are human causations, not a free choice on the part of the animal. “A study found that dogs that lived in households where there were kerosene heaters were more likely to have nasal cancer. Another cancer found in both humans and dogs is lymphosarcoma. One study found that animals exposed to pesticides were more likely to get that type of cancer. Some of it is exposure to chemicals, toxicity, etc,” Forrest says.

There is another gigantic difference between pet and pet owner and their cancer treatment. “The difference between the animal side and the people side is that with the people side you are treating one species and one breed, so to speak—humans,” Forrest says. “The other thing about humans is health insurance. It’s a much bigger outfit. Not only are there MDs, but there are therapists and physicists—there is a whole group of people. Whereas on the veterinary side, we have to wear all those hats. We do the treatment planning and we deliver the therapy and so on.” But Forrest notes that, differences aside, in addition to types and symptoms of cancer being similar in pet and pet owner, there are similarities in treatment.

Large Advances, Small Patients
Radiation therapy is a large advancement for animals with cancer as opposed to the former option, euthanasia. Treating cancer in animals brings a new meaning to PET/CT. “If an animal comes in with a tumor, you may want to image it with CT or you may want to use ultrasound and you would want to get a biopsy to figure out the histology and tumor type,” Forrest says. “Then would be staging of the tumor—not only do we want to know what kind of tumor it is, but we want to know where it is—is it local or has it spread? We may do CT of the thorax to see if it has spread to the lungs. We often will do abdominal ultrasound to see if it has spread to the organs. We have basically the same imaging modalities they use in human medicine.

“There are a few that are still using cobalt units,” Forrest says. “But more and more of us are getting linear accelerators, and even some are getting more advance equipment with multileaf collimators and have the ability to do IMRT [intensity-modulated radiation therapy].” Additionally, other options similar to human cancer treatments are performed on pets—everything from nuclear medicine and MRI to brachytherapy, which has been used to treat nasal tumors in dogs and fibrosarcomas in cats.

Specialty Facilities
However, much of this technology and equipment is most likely not typical to your neighborhood vet. University and teaching hospital facilities generally have more funding and thus better equipment and more cutting-edge treatment options. “There are specialty practices and what we try to encourage is getting your local veterinarian to possibly go as far as doing some of the staging and then referring pet owners to a specialty setting—similar to if you went to the doctor and they suspected a tumor you would be referred to a specialist or an oncologist,” Forrest says.

While many animals are susceptible to cancer and the list of animals people keep as pets is growing, not every pet is a good match for radiation therapy in this type of setting. “Most of what we do are cancers in what we call companion animals—dogs and cats,” Forrest says. “Soft tissue tumors and nasal tumors in dogs are very common. We’ve done rabbits with thymomas and tumors of the thymus in the thoracic cavity,” she says. “When we do therapy, the animals have to be under general anesthesia because they aren’t going to sit still long enough on the table. We have to set them up and have them be still and we have to leave the room while the therapy is administered. It’s akin to doing pediatric radiation therapy.”

Radiation therapy for small and domestic animals is one thing, but large animals and zoological residents are a horse of another color. An oncology specialist at Southern Arizona Veterinary Specialty & Emergency Center noted that zoological species and other exotic animals that are candidates for radiation therapy generally are not treated at veterinary centers but rather at the zoological parks or wildlife reserves that house the animals. Horses and bovine are accommodated at many animal hospitals and at UW in the large animal hospital, but “horses on a whole don’t get a lot of cancers compared to dogs and cats. It’s a much smaller population that undergoes radiation treatment,” Forrest says.

Treatment Course
According to UW’s Web site, the therapy itself generally consists of a series of treatments over two to five weeks, depending on the size and location of the cancer, the pet’s general health, and the type of cancer. During treatment, animals are anesthetized and placed on a treatment table. A large mechanical arm that contains the radiation source rotates around the patient and irradiates the tumor from different directions. High-energy radiation is directed at the cancer and some normal tissue around it. A purple “x” marks the spot on a patient’s body where radiation therapy will be directed so the same area is treated each time.

As in humans, many animals develop skin changes in the area being treated. Also, the skin and fur may be affected near the end of, or after, radiation therapy. “We had a black cat that had a vaccine-associated sarcoma, and we aren’t completely sure of the complete etiology, but they see these tumors at sites where vaccines have previously been given and something affects the cells to cause a secondary sarcoma to arise there,” Forrest says. “The cat had the tumor excised and we followed him up with radiation therapy. Since we were delivering the therapy to a black cat in the treatment field, the hair grew back white. He did quite well. We actually don’t always do such a good job with this tumor because it can be quite aggressive. But he did very well and we cured him, so these are the exciting, fun stories when you actually can give an animal a second chance and cure them. My favorites are the ones that are just troopers who come in for their radiation and are wagging their tails.”

The Cost of Cancer
Veterinary radiation therapy, for the most part, is like the pets being treated—warm, fuzzy, and feel good, or at least feel better. One caveat is payment. Pet cancer can take a toll on the pet, the family, and the wallet. Since the cost of radiation therapy is often many times over the initial cost of the pet itself, refusal of treatment is an option with which owners need to grapple. Although pet insurance is rising in popularity, radiation therapy is still a blow to the wallet, starting at nearly $900. “Unless we have funded trials, the owners will be paying for the therapy. It can be around $5,000 depending on the treatment regimen and what is involved,” Forrest says. “A lot of these owners are committed caregivers and they feel their pet is part of the family. It’s a decision owners will have to wrestle with, but a lot of the owners do go forward with it. I don’t think they look at it as being a ‘free animal’ from the Humane Society and they feel it is worth improving the pet’s quality of life.”

But like any other branch of medicine, there is not always a happy ending. Despite best efforts, sometimes other options must be examined. “Sometimes it happens, sometimes we work with them. Depending on how far advanced the tumor is, we advocate curative treatment, but in some instances, we do palliative care focused on easing pain while discussing euthanasia. It really depends on each scenario, it varies owner to owner.”

The owner of four cats—18-year-old Willie, who has heart disease, and a handful of refugees she has collected along the way—Forrest has close relationships with her animals. “They are my family,” she says. And as a veterinarian and pet owner, she is unsure what she would do when faced with the decision should radiation therapy need to be addressed within her feline family. “I think it would depend. If they were otherwise healthy, yes.” However if Willie in his old age was diagnosed with cancer, she says, “Being 18 he’s a pretty old cat [the median age is 12 to 15], but he’s pretty healthy aside from his heart disease. It would depend on their health and what the cancer was, but it would be hard not to [treat].”

— Meghan A.T.B. Reese is assistant editor of Radiology Today.